In the specifications of Long Term Evolution (including LTE and LTE-Advanced) stipulated in the 3rd Generation Partnership Project (3GPP), a packet data convergence protocol (PDCP) layer is stipulated above the radio link control (RLC) layer (refer to Non-Patent Document 1).
When the PDCP is re-established in cases such as handover to another cell or reconnection to the cell (hereinafter, HO/reconnection), a mobile station (UE) transmits to a radio base station (eNB) a PDCP status report. The eNB instructs the UE whether to transmit the PDCP status report or not for each radio bearer.
The object of the PDCP status report is to notify the eNB of reception status of a protocol data unit (PDU) in the PDCP level in the UE and avoid double transmission of the PDU in the PDCP level. In other words, to avoid double transmission of the PDCP PDU after an HO/reconnection.
The PDCP status report includes a First Missing sequence number (FMS) and a bitmap. The FMS indicates a sequence number (SN) of the oldest non-received PDCP PDU. The bitmap indicates reception status of the FMS and PDCP PDUs subsequent to that FMS by using 0 (NG) or 1 (OK).
In Release-13 of the 3GPP, how to extend the limitation of maximum 5 component carriers (CC) used in the carrier aggregation (CA) stipulated in Release-12 LTE up to a maximum of 32 CCs is being studied.
Considering the increase in peak throughput following such extension, extension of the SN length of the PDCP PDU has been agreed upon (refer to Non-Patent Document 2).